The immobilization antigen (i-antigen) of Paramecium is a surface protein which is distributed over 90 percent of the cell membrane. While a clonal population of cells generally expresses only one serotype at any given time, individual cells are capable of expressing as many as twelve different antigens. Serotype expression, as well as the phenomenon of mutual exclusion, is under the control of a complex interaction of genes, cytoplasm and cellular environment. Discrepancies in various physical and chemical measurements have arisen because methods used to purify the i-antigen have failed to remove trace amounts of contaminating proteins including some with limited proteolytic activity. We have developed a method of purification for i-antigens A and D using immunoaffinity chromatography. The product of this purification appears to consist of unequal subunits following reduction and alkylation. In addition, fractionation of the product derived from "pure" clones of serotype A and D revealed the presence of "secondary" i-antigens which are not detected in immobilizing tests on live cells. BILIOGRAPHIC REFERENCES: Steers, E. and Davis, Jr. R.H.: A Reexamination of the Immobilization Antigen from P.tetraurelia by Immunoaffinity Chromatography. In Press. 1977. Steers, E. and Davis, Jr. R.H.: A Reexamination of the Immobilization Antigen from P. aurelia. Comparative Biochem. & Physiol. 56B, 195-197, 1977.